diff --git a/antora-playbook.yaml b/antora-playbook.yaml
index f1564d0..b5d9a0a 100644
--- a/antora-playbook.yaml
+++ b/antora-playbook.yaml
@@ -72,12 +72,13 @@ ui:
         <!-- `@asciidoctor/tabs` extension styles -->
         <link rel="stylesheet" href="{{{uiRootPath}}}/css/vendor/tabs.css">
         <style>
-          /* Default `h4` and `h5` are smaller than the normal text, fix header font sizing: */
+          /* Default `h4`, `h5`, and `h6` are smaller than the normal text, fix header font sizing: */
           .doc h1 { font-size: 1.9rem; }
           .doc h2 { font-size: 1.7rem; }
           .doc h3 { font-size: 1.5rem; font-weight: 400; }
           .doc h4 { font-size: 1.3rem; font-weight: 500; }
           .doc h5 { font-size: 1.1rem; font-weight: 500; text-decoration: underline; }
+          .doc h6 { font-size: 0.9rem; font-weight: 500; text-decoration: underline; }
           /* Default `code`, `pre`, and `.colist` (source code annotations) fonts are too big, adjust them: */
           .doc .colist>table code, .doc p code, .doc thead code { font-size: 0.8em; }
           .doc pre { font-size: 0.7rem; }